Acoustic device



July 30, 1946. L. e; 'BOSTWICK ACOUSTIC DEVICE Filed June 7, 1940 3 Sheets-Sheet 2 INVENTOR L.G. BOSTW/CK BV MA-MhM' A 7'7'ORNEY July 30, 1946. G, wlcK 2,404,784

ACOUSTIC DEVICE Filed June '7, 19,40 3 Sheets-Sheet 3 FIG 6 INVENT O R y L.G.BO$TW/C K aims/M AT Z'ORNE V Patented July 30, 1946 UNITED STATES PATENT OFFICE ACOUSTIC DEVICE Application June 7, 1940, Serial No. 339,261

10 Claims. 1

This invention relates to electroacoustic transducers suitable for underwater operation and more particularly to submarine sound receivers and transmitters.

One object of this invention is to provide an electroacoustic transducer adapted for underwater operation and having abroad and uniform frequency range.

Another object of this invention is to provide a submarine type sound translating device, having an electromechanical system, that is both eflicient and capable of withstanding high underwater pressures.

One feature of this invention resides in a submarine sound translating device having a vibrating system of low effective mass and stiffness.

In accordance with another feature of this invention, the translating device has a resonant frequency low enough to be critically damped by mechanical motional resistance and by simple damping means employing air or other gaseous material.

A further feature of this invention involves the use of a thin diaphragm and means for balancing the static and low frequency pressures on opposite sides thereof.

An additional feature of this invention resides in a thin domed diaphragm of a diameter sufficient to cooperate with an efficient magnetic system, but having only a restricted portion of its area subjected to water pressure, whereby the water impedance is maintained at a desired value.

Other and further objects and features of this invention will appear more clearly and, fully from the following description of illustrative embodiments thereof taken in connection with the appended drawings in which:

Fig. 1 is a sectional View of a receiver illustrating one embodiment of the invention;

Fig. 2 is a section taken on lines 22 of Fig, 1;

Fig. 3 is a view partially in section of a receiver like that of Fig. 1 including means for equalizing the pressures on opposite sides of the receiver diaphragm;

Fig. 4 is an enlarged fractional view of the diaphragm showing details thereof;

Fig. 5 is a section on line 55 of Fig, 4; and

Figs. 6 to 11, inclusive, show respectively three modifications of a portion of the acoustic elements of the device.

Referring now to the drawings, the receiver designated generally by reference character It comprises an annular magnetic structure and a domed diaphragm. The magnetic structure comprises a center pole H having a flange I2 at one 2 end; a ring-shaped permanent magnet l3 secured to flan e I2 and a plate pole I4 attached to the magnet I3. Members I 2', I3 and I4 may be secured together by welding or in any other suitable rmanner. The diaphragm I5, of bronze or other suitable material, has a flange I6 which is clamped between rings I7 and I8 secured to plate pole I4 by fastening means such as screws I9. A depending flange portion 20 of the diaphragm supports the moving coil 2| in the annular airgap 22 between members I I and I l. A rin 23' of flexible material, e. g. rubber or the like, or metal, is secured to the diaphragm I5 intermediate its center and flange I6. The outer portion of ring 23 may be secured to a ring 25, as by cementing. The ring which may be of brass or other non-magnetic material, may be fastened to plate pole I4 by means of screws 26. When the receiver is secured to the hull or skin 24 of the" vessel, as by bolts I0, the ring 23 is firmly clamped between the hull and ring 25.

That portion of diaphragm I5 lying between the ring 23 and the flange It may contain a plurality of orifices 2'! which maybe provided with 25 flanges 28. Acoustic resistance material, such as felt or silk fabric 29, may be secured over the orifices 21. The flanges 28 may extend inwardly as shown or outwardly, if desired. These flanges 28 are to stiffen the diaphragm around the orifices 2i and may be dispensed with where such stiffening is not necessary. The orifices, flanges and resistance material have been shown somewhat exaggerated as to size in the interest of clarity of illustration.

On the concave side of the diaphragm I5 and secured to center pole II is a member 30 which may be of brass or other non-magnetic material. The outer surface of member conforms to the inner surface of the diaphragm and is spaced slightly therefrom. Member 30 serves as a stop to prevent inward collapse of the diaphragm due to extreme pressure or sudden surges caused by waves, underwater explosions, etc. This member 30 also is dimensioned to control the volume of the chamber 3| behind the diaphragm in accordance with desired acoustic characteristics. A plurality of orifices 32 in member 3!] connect the chamber 31 to a passage 33 in the centerpole I I. Other means than the orifices 32 may be emplayed to afford communication between chamber 3| and passage 33. For example, a plurality of radial slots 'Ili may be formed in the bottom of member 33, before it is secured topole-piece II as shown in Figs. 6 and 7; member 30 may be spaced slightly from pole-piece I I and supported 3 on a plurality of studs or pins 12 as shown in Figs. 10 and 11 or by a notched or discontinuous flange (I as shown in Figs. 8 and 9; or some suitable combination of the foregoing expedients may be used.

Portions of the flexible ring 23, ring member 25, plate pole l4 and diaphragm I5, define an annular chamber 34 on the convex side of the diaphragm. The chambers 3| and 34 may be connected through the acoustic resistance material 29 covering orifices 21.

The stiffness of flange it of the diaphragm may be reduced by means of a plurality of overlapping arcuate slots 36, as may be seen in the detail views of Figs. 2, 4 and 5. If it is desired to prevent leakage through slots 36, a thin membrane 31 of suitable material, such as a cellulose plastic may be secured, as by cementing, to flange I6 over said slots. This membrane 31 may be placed on the under or back side of flange 16 as shown is Fig. 5 or on the top or front side of said flange, if desired. In place of membrane 31 a plurality of strips of plastic material may be employed one for each slot 36. If leakage through slots 36 is indicated by the design requirements of the device, they may be left open, or only partially covered as the situation demands.

Connections may be made to coil 2| by suitable means, such as flexible conductors 5B, which may be brought out through the clamping rings l1 and I8. These conductors may be connected to leads 51, which may be brought out through the ring 25, being sealed in to prevent leakage from the chamber 34.

A ring or washer 35 of rubber or other suitable material may be secured to center pole H and plate pole 14, to close the inner end of the air-gap 22.

The diaphragm may be protected from damage by submerged objects and the like, by means of a rid or screen 89. The member 60 may be secured to the hull of the vessel by any suitable means, such as screws or bolts 6! The orifices 62 in the screen are made large enough so that they have no appreciable acoustic eifect on receiver operation.

In the sectional view, Fig. 2, which shows the convex surface of the diaphragm IS, the resilient ring 23 has been omitted in order that details therebehind may be more clearly shown. The region of attachment of this ring to the diaphragm is shown by dot-dash lines on the dome portion of the diaphragm.

Referring to Fig. 3, the passage 33 of the receiver i is connected by a tube 40 to a chamber 41. The chamber 4| may comprise a bellows 42 of bronze or the like, secured in a housing 43 by a cover 44. The bottom portion of housing 43 may be connected to the water outside of the vessel by means of a pipe 45 suitably secured to the hull at orifice 46 as by flange 41.

The water pressure outside of the vessel is transmitted to receiver chamber 3| and hence to the back of the diaphragm l via orifice 46, pipe 45, interior of housing 43, bellows 42, chamber 4 I, tube 40, passage 33, orifices 32 to chamber 3|. Since the front of the diaphragm i5 is also subjected to the Water pressure, the foregoing means provides for equalization of pressure on the opposite sides thereof. The impedances of the tubes, pipes, chambers, etc., comprising the equalizing system, are such that it acts as a lowpass filter. The static pressure and low frequency pressures, such a are caused by waves, etc., are thus equalized. Pressures at sound frequencies,

on the other hand, are sufiiciently attenuated to prevent their equalization, allowing actuation of the diaphragm in response thereto.

The chambers 34, 3|, 4| and the various passages interconnecting said chambers may be filled with a suitable fluid, preferably gaseous material, such as air or nitrogen.

When the diaphragm l5 vibrates, the fluid in chambers 3| and 34 is forced back and forth through the acoustic resistance means 29. By employing air or a gas as the fluid, mechanical damping which is independent of temperature is thus afforded. The chambers 3| and 34 and the acoustic resistance material 29 are proportioned to give the required damping effect at low frequencies near resonance. Venting of the diaphragm through the orifices 21 reduces chamber stiffness and permits a low resonant frequency.

In a device of this type the diaphragm should be big enough so that a suitable magnetic system may be associated therewith; but not so large that the mechanical impedance of the water too greatly exceeds that of the vibrating system, which would result in inefiicient conversion. With the type of instrument disclosed, having a low resonant frequency, the velocit of the vibrating system at high frequencies will be determined by the mass reactances KB. of the water and X of the vibrating system as a whole, respectively. Thus,

where f=the force produced by the sound effective at the diaphragm,

Since XR=K2T3 The water mass then should be twice the mass of the vibrating system to obtain maximum velocity. This desirable relationship can be realized with a light and efiicient vibrating system by restricting the area of the diaphragm exposed to the water.

The foregoing specific description has, for convenience, been restricted to a submarine sound receiver. However, a transducer such as is contemplated by this invention may also be employed as a submarine sound transmitter. When used as a sound transmitter or radiator, the damping resistance at the diaphragm orifices 27 should be made low, the diameter of said orifices large, and the volume of the chamber of the concave side of the diaphragm large as compared with a receiver. These changes will result in a lower resonance frequency above which the Velocity of the diaphragm will Vary inversely with frequency and closely compensate for the quadratic increase with frequency in the radiation or loading resistance of the water. Substantially uniform acoustic output will then result.

Although specific embodiments of this invention have been shown and described, it will be understood that modifications may be made therein without departing from the scope and spirit thereof as defined by the appended claims.

What is claimed is:

1. A submarine sound receiver comprising a structure including magnet means, a domed diaphragm attached to said structure, an electrical current conductor secured to the diaphragm and located in the field of said magnet means, stop means secured to said structure and spaced from the inner surface of the diaphragm to define therewith a chamber, flexible means secured to the outer surface of the diaphragm to restrict the area thereof subjected to exterior pressure, said flexible means, diaphragm and structure defining a second chamber, and acoustic resistance means, associated with the diaphragm and connecting the two chambers.

2. An acoustic transducer comprising a structure including means for producing a magnetic field, a domed diaphragm secured to said structure and defining therewith an acoustic chamber, an electrical current conductor attached to the diaphragm and positioned in said field and resilient means secured to said structure and diaphragm for restricting the area of the diaphragm subjected to sound pressure, a portion of the resilient means and the diaphragm defining with said structure a second acoustic chamber, said diaphragm having acoustic resistance orifices therein connecting th two acoustic chambers.

3. An acoustic device for operation in a sound transmitting medium more dense than air, comprising a structure including magnetic field producing means, a non-magnetic diaphragm having a stiff, convex central portion and a resilient outer portion connected to said structure, a sound current coil secured to the diaphragm in cooperative relation with the magnetic field producing means, means for restricting the area of the diaphragm subjected to the pressure of the sound transmitting medium, acoustic impedance chambers on opposite sides of the diaphragm, said restricting means cooperating with a portion of the diaphragm to define one of said impedance chambers, and acoustic resistance means secured to the diaphragm and connecting the impedance chambers.

4. An acoustic device for underwater operation comprising a structure, an outwardly convex, domed diaphragm cooperating with said structure to define a chamber, means in front of said diaphragm for restricting water pressure to a portion thereof, said means cooperating with the structure and diaphragm to define another chamber, and means defining restricted passages between said chambers.

5. -.A submarine sound translating device comprising a structure, an outwardly convex, domed diaphragm resilientl attached to said structure and defining with said structure a chamber, a resilient ring secured to the convex surface of the diaphragm dome and to the structur for confining water pressure to a restricted portion of the diaphragm, a second chamber behind said resilient means and outside of the diaphragm dome, and acoustic resistance means on said diaphragm and interconnecting the chambers.

, 6. A submarine sound receiver comprising a structure including magnetic means having polepieces defining an annular air-gap, a diaphragm ofnon-magnetic material and having a thin, convex portion, a resilient peripheral flange, and a depending flange, a sound current coil mounted on said depending flange, said peripheral flange secured to a pole-piece to position the coil in the air-gap, said diaphragm and structure defining a chamber, and a ring of resilient material secured at its inner edge to the convex side of the diaphragm intermediate the center and the peripheral flange and having its outer portion secured to said structure, a portion each of said ring, diaphragm and structure defining a second chamber, said portion of the diaphragm having acoustically resistant orifices connecting the two chambers.

structure including magnetic means having a center pole, and a plate pole spaced therefrom to define an annular air-gap, a diaphragm of nonmagnetic material having. a thin, central dome portion and a resilient supporting flange secured thereof in contact with water pressure, a portion each of the resilient ring, the diaphragm and the structure defining a second chamber, and acoustic resistance means associated with orifices in said diaphragm, and connecting said chambers.

8. A submarine sound receiver comprising a magnet structure including a pole-piece, an outwardl convex diaphragm of thin non-magnetic material havin an electrical current conductor secured thereto in cooperative relation to said pole-piece, resilient means secured to said diaphragm and associated with said structure to confine a restricted portion of said diaphragm to external pressure and to define with said diaphragm and structure a chamber on one side of said diaphragm, a chamber on the other side of the diaphragm, and acoustic resistance means connecting said chambers together through said diaphr m.

9. A submarine sound receiver comprising a structure including magnetic means having a center pole, and a plate pole spaced therefrom to define an annular air-gap, a diaphragm of thin nonmagnetic metal having a central, outwardly convex dome portion and a resilient supporting flange portion secured to said plate pole, a sound current coil secured to the diaphragm and positioned in the air-gap, an outwardly convex stop member secured to said center pole, spaced from the dome portion of the diaphragm and defining therewith a chamber, a ring of rubber-like material secured to said structure and to the convex side of the diaphragm, intermediate the center and the supporting flange thereof, to limit a portion only of the diaphragm dome to external pressure, a portion each of the ring, the structure and the diaphragm defining a second chamber, behind said ring and outside of the diaphragm, the portion of the diaphragm between the ring and flange containing perforations through which said chambers are interconnected, and acoustic re- 7. A submarine sound receiver comprising a sistance material secured to th diaphragm over said perforations.

10. A submarine sound translating device com prising a structure, an outwardly convex, domed diaphragm resiliently attached to said structure and defining with said structure a chamber, flexible means secured to the convex surface of the 8 diaphragm dome and to the structure for confining water pressure to a restricted portion of the diaphragm, a second chamber behind said flexible means and outside of the diaphragm dome, and means including acoustic resistance interconnecting said chambers.

LEE G. BOST WICK. 

